They could have been on a family outing, a small group of adults and children ambling along the banks of a wide river estuary on the Norfolk coast.

But the footprints they planted in the mud at Happisburgh were left 800,000 years ago - the oldest marks made by human feet ever discovered outside Africa.

Scientists are now trying to understand the full implications of the find, the first direct evidence of our earliest ancestors in northern Europe.

With astonishing luck, researchers were able to make digital records of the prints two weeks before they were destroyed by encroaching sea tides.

The footprints were exposed at low tide as heavy seas washed away beach sands to reveal a series of elongated hollows cut in the compacted mud.

"At first we weren't sure what we were seeing," said Dr Nick Ashton, of the British Museum in London. "But as we removed any remaining beach sand and sponged off the seawater, it was clear that the hollows resembled prints, perhaps human footprints, and that we needed to record the surface as quickly as possible before the sea eroded it away."

Analysis revealed a range of adult and juvenile feet, possibly five people in all. In some cases there was evidence of the heel, foot arch and even toes.

Working on the basis that in most present and past populations foot length is roughly 15% of height, the scientists calculated how tall the ancient walkers were likely to have been.

Their heights varied from three feet to 5ft 7ins. The tallest individual had feet that would have fitted a modern size eight shoe.

"This height range suggests a mix of adults and children with the largest print possibly being a male," said Dr Isabelle De Groote, a member of the team from Liverpool John Moores University.

A report on the discovery appears in the online journal Public Library of Science ONE.

At the time the footprints were made, Britain was linked by land to continental Europe. The Happisburgh site would have been on the banks of a wide estuary several miles from the coast.

The surrounding landscape then was very different from that of modern Norfolk, with deer, bison, mammoth and hippo grazing along the river valley.

As well as abundant meat, the ancient humans would have had access to edible plant tubers, seaweed and shellfish .

Mammalian fossils unearthed at Happisburgh include a species of woolly mammoth, an extinct horse and early forms of vole.

Until now stone tools and animal bones have been the only relics of humans living in northern Europe so long ago.

Who left the footprints remains a mystery, but they may have been related to Homo antecessor (Pioneer Man), a human species known to have lived in Spain at around the same time.

Professor Chris Stringer, head of human origins at the Natural History Museum, London, said: "These people were of a similar height to ourselves and were fully bipedal.

"They seem to have become extinct in Europe by 600,000 years ago and were perhaps replaced by the species Homo heidelbergensis. Neanderthals followed from about 400,000 years ago, and eventually modern humans some 40,000 years ago."

The Norfolk prints are more than twice the age of the previous oldest human tracks found in Europe, the so-called "Devil's Footprints" left in volcanic ash in the Campanian plain of southern Italy which date back 345,000 years.

The oldest footprints from a human-like species ever found were left at Laetoli, Tanzania, some 3.5 million years ago.

Footprints from a people who appear to have walked in a similar way to modern humans were found at Lleret in Kenya in 2009 and are around 1.5 million years old.